In many instances it is important to be able to determine the distribution of vibrations across an object. Vibrations may, for example, cause too high a load on critical portion of a structure, or they may create undesirable noise. The present invention permits an accurate determination of the actual oscillations and their distribution for an object during its operation.
There are some techniques and methods today that may be used for such mapping. We will now describe these techniques briefly and discuss some of their strong and weak points:
(1) Local measuring methods.
Local measuring methods mean measurements based on one or a number of single-element sensors (sensors/detectors) which measure displacements of gas (air) as well as solids. They comprise measurements by microphone (sound measurements), accelerometer, strain gauge, eddy current, photo multiplier tube, or semiconductor diode, etc. The methods are generally characterized by having high time resolution and the ability to follow rapid time variations. However, they are also characterized by the fact that only the resulting displacement of one local area is read. This means that in order to make a map/image of the distribution of vibrations in an object, it is necessary either to construct a matrix of sensors or to sweep one or more sensors across a large area. In both cases we end up with time consuming and complicated measurements and the spatial accuracy (determination of the positions of the vibrations) aimed at may be difficult to achieve.
(2) Image-forming measuring methods.
Image-forming measurements encompass all kinds of image-reading measurements wherein the object is illuminated and the object vibrations are read using measurements taken of an entire image. The best known techniques have names formed with the words moire, speckle, or interferrometry.
The image-forming methods are characterized by the fact that it is easy to achieve the desired image-forming effect, e.g. by using image-forming optics (lenses, mirrors, etc) combined with "image-detectors" such as photographic film, TV-camers, etc. This results in techniques having great spatial accuracy (spatial resoluton). In order to achieve this spatial accuracy we must pay by having limited time resolution. The repetition frequency of the image recording is generally too low, and we end up with almost insurmountable amounts of data to be processed.
With the aid of the present invention it is possible to map vibrations by carrying out an efficient data-reduction and by combining the advantages of both the local and the image-forming methods of measuring; namely high time resoultion on the local side combined with high spatial resolution on the image-forming side.